The magnetic recording disk in a conventional drive assembly typically consists of a substrate, an underlayer consisting of a thin film of chromium (Cr) or a Cr alloy, a cobalt-based magnetic alloy deposited on the underlayer, and a protective overcoat over the magnetic layer. A variety of disk substrates such as NiP-coated AlMg, glass, glass ceramic, glassy carbon, etc., have been used. The microstructural parameters of the magnetic layer, i.e., crystallographic preferred orientation (PO), grain size and magnetic exchange decoupling between the grains, play key roles in controlling the recording characteristics of the disk. The Cr underlayer is mainly used to control such microstructural parameters as the PO and grain size of the cobalt-based magnetic alloy. The PO of the various materials forming the layers on the disk is not necessarily an exclusive orientation which may found in the material, but is merely the dominant orientation. When the Cr underlayer is deposited at elevated temperature on a NiP-coated AlMg substrate a [100] preferred orientation (PO) is usually formed. This PO promotes the epitaxial growth of [1120] PO of the hcp cobalt (Co) alloy, thereby improving the in-plane magnetic performance of the disk. The [1120] PO refers to a film of hexagonal structure whose (1120) planes are predominantly parallel to the surface of the film. Likewise the [1010] PO refers to a film of hexagonal structure whose (1010) planes are predominantly parallel to the surface of the film. Since nucleation and growth of Cr or Cr alloy underlayers on glass and most non-metallic substrates differ significantly from those on NiP-coated AlMg substrates, media fabricated on glass substrates often have larger noise compared with those made on NiP-coated AlMg substrates under identical deposition conditions. It is for this reason that the use of an initial layer on the substrate (called the seed layer) is necessary. The seed layer is formed between the alternate substrate and the underlayer in order to control nucleation and growth of the underlayer which in turn affects the magnetic layer. Several materials have been proposed for seed layers such as: Al, Cr, CrNi, Ti, Ni.sub.3 P, MgO, Ta, C, W, Zr, AlN and NiAl on glass and non-metallic substrates. (See for example, "Seed Layer induced (002) crystallographic texture in NiAl underlayers," Lee, et al., J. Appl. Phys. 79(8), Apr. 15, 1996, p.4902ff). In a single magnetic layer disk, Laughlin, et al., have described use of an NiAl seed layer followed by a 2.5 nm thick Cr underlayer and a CoCrPt magnetic layer. The NiAl seed layer with the Cr underlayer was said to induce the [1010] texture in the magnetic layer. ("The Control and Characterization of the Crystallographic Texture of Longitudinal Thin Film Recording Media," IEEE Trans. Magnetic. 32(5) September 1996, 3632).
The improvement in signal to noise ratio (SNR) of the thin film disk media remains as one of the major challenges in high density recording technology. A variety of approaches such as choosing a low noise alloy, designing an appropriate underlayer, tailoring of the deposition parameters, and lamination of the magnetic layer have been suggested to reduce the media noise. A laminated disk has two or more magnetic layers separated by a spacer layer. For example, Ahlert, et al. in commonly assigned U.S. Pat. No. 5,051,288 describe laminated disks with AlMg/NiP substrates and up to six layers of CoPtX or CoNiX alloys separated by Cr, CrV and Mo layers.
Laminating the magnetic layer of a thin film disk is known to reduce the media noise, but laminated media typically exhibit a bi-modal switching behavior due to the fact that the coercivity (Hc) of the stacked magnetic layers can be significantly different. Laminated media with optimum performance should exhibit only one type of switching behavior, which means the stacked magnetic layers should have very similar Hc. For most magnetic alloys employed in thin film disk technology Hc is a function of deposition temperature, i.e., Hc increases with substrate temperature. The sputtering systems used for the volume production of magnetic disks provide the capability to preheat the substrates, but as the sputtering process progresses over time the temperature of the substrate declines. Thus, when laminated magnetic layers are sputtered on a preheated substrate, the second layer is deposited at a lower temperature and will typically have a lower Hc. The reduced Hc in the second layer at least contributes to a deviation (kink) in the smooth slope of the hysteresis loop around the zero remanent magnetization state. FIGS. 3a and 3b show typical hysteresis loops of a single magnetic layer (Cr/CoPtCrTa) and a two layer laminated magnetic film (Cr/CoPtCrTa/Cr/CoPtCrTa), respectively. The kink in the hysteresis of the laminated film is clearly seen. The existence of this kink implies that the film has two switching characteristics which will deteriorate the recording performance of the disk at high recording densities. Thus, it is desirable to design a laminated media without bi-modal switching behavior to eliminate this kink, or to reduce it as much as possible.